The invention is directed to a method and apparatus for the compensation of dynamic error signals of a chopped Hall sensor.
Hall sensors are magnetic field sensors that are based on the Hall effect. The Hall effect arises when the electrons of an excitation current move through a Hall sensor in a transverse magnetic field. As a result of the Lorentz force acting on the electrons, an electric field forms approximately transverse to the direction of current, this corresponding to an electrical voltage that is referred to as a Hall voltage.
In the practical application of Hall sensors, it is required for exact magnetic field measurements that unwanted offset voltages be eliminated from the Hall voltage or that these be at least suppressed as well as possible. Examples of offset voltages are the ohmic and the capacitive homopolar components of a Hall sensor or sensor element.
Hall sensors are usually manufactured of a semiconductor upon application of traditional methods of semiconductor technology. A plurality of Hall sensor elements that form a Hall sensor together with electronics for signal processing are often manufactured as an integrated circuit and also are mounted on a chip carrier and in a housing made of plastic.
As a result of production tolerances and the mounting, however, bendings can occur in the semiconductor crystal of the integrated Hall sensor circuit, these, in particular, leading to the aforementioned ohmic homopolar components.
Various methods are known for the compensation of offset voltages. Given high-precision Hall sensors, for example, what is referred to as quadrature switching or a spinning Hall switchover (also called spinning Hall principle) is utilized. Hall sensors wherein these methods are applied are also referred to as chopped Hall sensors. U.S. Pat. No. 5,406,202 discloses such a chopped Hall sensor.
Given chopped Hall sensors, the direction of the (excitation) current through a Hall sensor element is periodically changed. The direction of the offset voltages also changes as a result thereof; given a quadrature switching, only the operational sign of the offset voltages is changed, viewed mathematically. The offset voltages can therefore be ideally removed from the actual measured signal by means of an addition of the output voltages of a chopped Hall sensor. In practical embodiments, a switching with a prescribed (switching) frequency is periodically undertaken between various terminals of the Hall sensor elements via which the (excitation) current is impressed into the Hall sensor element.
FIG. 1 shows the effect of a 90° switchover given an approximately quadratic Hall plate having two terminal pairs as a Hall sensor element that is situated in a magnetic field B. The terminals of the two terminal pairs are respectively attached at a corner region of the Hall plate. An approximation model of the Hall plate is shown in the form of a resistance network. The Hall plate is situated in a magnetic field B. In phase 1, an excitation current I is impressed into terminals of a first terminal pair of the Hall plate. The output voltage obtained in phase 1 is shown in the diagram at the top in FIG. 1. It comprises the actual measured signal of the magnetic field that is referenced “magnetic field” and is the Hall voltage Vhall. It also comprises offset voltage Voffset of the Hall plate and of an operational amplifier for amplifying the output signal.
In phase 2, the direction of the current I through the Hall plate is turned by 90° in that this is impressed into the terminals of a second terminal pair of the Hall plate. As a result thereof, the operational sign of the offset voltages Voffset changes when—as shown on the basis of the resistance network—the output voltage is measured at the terminals via which the current I was impressed in phase 1.
FIG. 2 shows a Hall sensor element with a plurality of terminals that are arranged approximately equally spaced at the edge of the Hall sensor element. This Hall sensor element is an octagonal Hall plate. A terminal is attached to each corner. The illustrated eight terminals form four terminal pairs via which an excitation current can be impressed into the Hall plate or a Hall voltage can be taken.
Given this Hall sensor element, the excitation current I can be cyclically impressed in the arrow direction. As a result thereof, the payload signal is constantly present over a plurality of clock cycles, whereas error signals average out. The diagram at the top in FIG. 2 illustrates this, and shows the time curve of the output voltage V cyclically taken at the terminals that comprises the Hall voltage Vhall and offset voltages Voffset. This operating mode of a Hall sensor is referred to as a spinning Hall principle.
However, parasitic capacitances must be recharged, these particularly occurring due to the terminals, leads to the terminals and switchovers or similar circuit elements. As a result thereof, dynamic error signals, especially in the form of spikes, occur in the measured signal. The dynamic error signals have a more and more disturbing influence on the measured signal the higher the switching frequency is. The share of the dynamic error signals compared to the actual measured signal then increases in the output signal of the Hall sensor element. The otherwise small errors due to offset voltages in chopped Hall sensors are increased again when processing high-frequency signals, i.e. given a high switching frequency.
Various methods have been previously disclosed in order to reduce the dynamic error signals. What is probably the simplest method for reducing the influence of dynamic error signals is a reduction of the chopper frequency. In most applications, however, the payload signal frequencies are fixed, so that a high circuit-oriented expense—usually an involved filtering—is required in order to separate payload and error signals. It has also already been proposed to utilize a slow inner and a fast outer chopper loop, which likewise requires a high circuit-oriented expense (see 2000 IEEE International Solid-State Circuits Conference 07803-5853-8/00 “TA 9.4 A CMOS Nested Chopper Instrumentation Amplifier with 100 nV Offset”, Anton Bakker, Kevin Thiele 1, Johan Huijsing). It is also known to use long chopper times in order to reduce the influence of dynamic error signals on the payload signal. Moreover, U.S. Pat. No. 5,621,319 discloses that a long dead phase be realized during the switching in order to optimally blank out the effects of the dynamic error signals.
Particularly given spinning Hall sensors, all of the above techniques only suppress dynamic error signals up to a specific mass and also diminish the time resolution as well as the bandwidth of the payload signal.